A. Field of the Invention
The instant invention is directed toward a conforming electrode and a method for using the conforming electrode for tissue ablation. In particular, the conforming electrode of the present invention comprises a plurality of flexible filaments or bristles which may be used for applying ablative energy (e.g., RF energy) to target tissue during the formation of spot or continuous linear lesions.
B. Background Art
It is well known that benefits may be gained by forming lesions in tissue if the depth and location of the lesions being formed can be controlled. In particular, it can be desirable to elevate tissue temperature to around 50° C. until lesions are formed via coagulation necrosis, which changes the electrical properties of the tissue. For example, when good or sufficiently deep lesions are formed at specific locations in cardiac tissue via coagulation necrosis, undesirable atrial fibrillations may be lessened or eliminated. The definition of “good” or “sufficiently deep” lesions depends at least to some extent on the procedure and may also depend on other considerations such as tissue characteristics. “Sufficiently deep” lesions means transmural lesions in some cardiac applications.
Several difficulties may be encountered, however, when attempting to form adequately-deep lesions at specific locations using some existing ablation electrodes. For example, when forming lesions with RF energy, high temperature gradients are often encountered in the vicinity of the electrode. At the edges of some existing electrodes are regions of very high current density, leading to large temperature gradients and hot spots. These “edge effects” may result in the formation of undesirable coagulum and charring of the surface tissue. For example, undesirable coagulum may begin to form when blood reaches around 80° C. for an appreciable length of time, and undesirable tissue charring and desiccation may be seen when tissue reaches around 100° C. for an appreciable length of time. There are two types of undesirable coagulum: coagulum that adheres to and damages the medical device; and coagulum blood clots or curds that may enter a patient's bloodstream, possibly resulting in other health problems for the patient. Charring of the surface tissue may also have deleterious effects on a patient.
As the temperature of the electrode is increased, the contact time required to form an adequately-deep lesion decreases, but the likelihood of charring surface tissue and forming undesirable coagulum increases. As the temperature of the electrode is decreased, the contact time required to form an adequately-deep lesion increases, but the likelihood of charring surface tissue and forming undesirable coagulum decreases. It is, therefore, a balancing act trying to ensure that tissue temperatures are adequately high for long enough to create deep lesions, while still preventing or minimizing coagulum formation and/or charring of the surface tissue. Active temperature control may help, but the placement of thermocouples, for example, is tricky and setting the RF generator for a certain temperature becomes an empirical exercise as actual tissue temperatures are generally different from those recorded next to the electrode due to factors such as convection and catheter design.
Thus, there remains a need for effective thermal control during ablation procedures.
Another difficulty encountered with existing ablation electrodes is how to ensure adequate tissue contact. Current techniques for creating continuous linear lesions in endocardial applications include, for example, dragging a conventional catheter on the tissue, using an array electrode, or using pre-formed electrodes. All of these devices comprise rigid electrodes that do not always conform to the tissue surface, especially when sharp gradients and undulations are present, such as at the ostium of each pulmonary vein in the left atrium and the isthmus of the right atrium. Consequently, continuous linear lesions are difficult to achieve. When forming lesions in a heart, the beating of the heart further complicates matters, making it difficult to keep adequate contact between the electrode and the tissue for a sufficient length of time to form a desired lesion. With a rigid electrode, it can be quite difficult to maintain sufficient contact pressure until an adequate lesion has been formed. This problem is exacerbated on contoured or trabeculated surfaces. If the contact between the electrode and the tissue cannot be properly maintained, a quality lesion is unlikely to be formed.
Catheters based upon a virtual electrode may address some of the difficulties, but these catheters often require high flow rates of conductive fluid (e.g., typically around 70 milliliters per minute) to maintain effective cooling for high-power RF applications. The introduction of a large amount of conductive fluid into a patient's bloodstream may have detrimental effects on the patient.
Brush electrodes containing flexible electrode filaments provide certain advantages, particularly in maintaining effective contact on irregular surfaces. Brush electrodes for ablation are known in the art but have been predominantly limited to arthroscopic procedures for vaporizing tissue. For example, Goble, et al., U.S. Pat. Nos. 5,944,715 and 6,780,180 disclose a number of examples of brush electrodes consisting of flexible metal fibers. However, arthroscopic ablation devices such as Goble are specifically designed to operate in articular joints filled with synovial fluids. The term “underwater surgery” is frequently used to describe these procedures. Under such conditions, large amounts of fluid may be introduced into the working surface without detrimental effects. However, as noted above, for catheters designed to work in vascular environments, the introduction of large quantities of conductive fluids into a patient's bloodstream is not desirable.
Previous disclosures have sought to address some of the difficulties associated with the use of brush electrodes in vascular environments. For example, in U.S. application Ser. No. 10/808,919, to which this application claims priority, an exemplary brush electrode comprised of about 2000 flexible filaments is disclosed. The filaments comprise part of a catheter with an outer sheath that surrounds the fibers and provides mechanical support for the flexible filaments. The sheath may also provide electrical shielding. The flexible filaments typically project a few millimeters from the distal end of the outer sheath. Conductive fluid such as saline may be provided to the distal end of the brush via interstitial spaces between the filaments. Additional embodiments may include hollow fibers, which can provide (and control) fluid to the target site via the additional paths through the fibers.
The advantages of using brush electrodes with small filaments and/or hollow passages in vascular environments are several. By providing many more filaments than in a typical arthroscopic brush, the total surface area of the working surface is greatly increased. Increased surface area dramatically increases the uniformity of energy transfer to a tissue site, greatly reducing the “edge effects” associated with traditional ablation electrodes as described above. Additionally, the increased surface area also dramatically increases the amount of heat transfer away from target tissue, allowing increased ablation times and reduced negative effects such as “charring” or coagulation of blood.
Advantageously, in vascular applications, the increased surface area available for cooling in brush electrodes with multiple small fibers means that the cooling fluid introduced into the fibers also transfers heat much more effectively. Thus, the total amount of cooling fluid required during a particular procedure may be greatly reduced and the undesirable medical consequences of introducing too much fluid into the vascular system at one time may be avoided.
Although the results associated with tests of brush electrodes such as those described above represent dramatic improvements in electrical and thermal performance, there remain additional design challenges to introducing these devices into vascular environments such as the heart.
It is desirable to minimize the risk that debris may be introduced to the body during surgery. One way to reduce this risk is to use instruments having fewer parts. In addition to minimizing the opportunity for debris to be introduced into the blood stream, using fewer parts allows for comprehensive accountability of parts when they are introduced and ultimately removed from the body.
Thus, there remains a need for conforming electrode catheters that address issues with existing designs and still permit the formation of uniform, transmural spot and continuous linear lesions on smooth or contoured surfaces in a manner that is safe and effective when used in vascular environments such as the heart.